I am trying hard to keep the ancillary contents of my new Lowpro backpack to a minimum, to give extra room for cameras and lenses. Sounds easy but modern digital camera come with a range of extras, some vital others less so. This is a collection of things I always carry. Most importantly I have spare batteries. I also have a few spare SD cards, not a surprise. I have found keeping a small USB universal card reader very handy. I usually have two 4GB USB keys on hand, one blank the other has a number of photo oriented portable apps and utilities. which are great if the only computer around is a cyber cafe. One big hassle is always cables, you seldom need them but then they are always tangled. I found this mess bag with a zip is a great way to keep my contents tidy and generally tangle free. The other important item is a small portable backpack style hard drive, with a 500GB capacity to keep backups updated (each day). At the end of the month I archive from it and clean it up to start a fresh for the next month.
I have an HTC android phone, which has become not only a camera in its own right but also a very useful support accessory. In particular it is the GPS source when I wish to geotag my photos but it also has a number of useful photography oriented apps. It lives in my pocket.
Not shown is my laptop, which does live in the Lowpro backpack as well, and all the charges and power supplies which I don’t carry around in the backpack but fit in my case when I travel.